The present invention relates to a process for preparation of water-soluble cellulose ethers, by alkalization and etherification of cellulose which has been activated with ammonia.
The preparation of cellulose ethers having uniform or different types of ether substituents is known (see, for example, "Ullmanns Encyklopaedie der technischen Chemie" [Ullmann's Encyclopedia of Industrial Chemistry], Volume 9, keyword "cellulose ethers", Verlag Chemie-Weinheim, 4th edition 1975, pages 192 et seq.), these being prepared, in general, either by (a) the principle of Williamson's ether synthesis, that is, by reacting cellulose with alkyl or aralkyl halides (with the consumption of a base) and/or by (b) as shown below, or by (c) reacting cellulose with activated reactants (in the presence of catalytic quantities of a base): ##STR1##
In these general equations:
Cell-O-H denotes, on the cellulose molecule, a hydroxyl group which is to be etherified, PA0 Hal denotes chlorine or bromine, PA0 R.sup.1 denotes an alkyl radical from C.sub.1 to C.sub.15, an aralkyl radical from C.sub.7 to C.sub.15, a carboxyalkyl radical from C.sub.1 to C.sub.3, a sulfonoalkyl radical from C.sub.1 to C.sub.3, a phosphonoalkyl radical from C.sub.1 to C.sub.3, a hydroxyalkyl radical from C.sub.1 to C.sub.6 or an N,N-dialkylaminoalkyl radical in which each alkyl group is from C.sub.1 to C.sub.3, PA0 R.sup.2, R.sup.3 denote hydrogen or an alkyl radical from C.sub.1 to C.sub.13, R.sup.2 being identical with R.sup.3 or different therefrom, PA0 BOH denotes a base, such as NaOH or a quaternary ammonium base, and PA0 R.sup.4 denotes an optionally N-substituted carboxamide or sulfonamide radical or a nitrile radical.
For preparing mixed ethers of cellulose, various etherifying agents are allowed to act simultaneously or sequentially on cellulose. For this purpose, reactions according to only one of the variants a to c indicated, but particularly reactions according to at least two of the variants are carried out. The following are examples of reaction products which can be prepared by variant (a): methyl cellulose (MC), benzyl cellulose (BC), carboxymethyl cellulose (CMC), sulfonoethyl cellulose (SEC), phosphonomethyl cellulose (PMC), or N,N-diethylaminoethyl cellulose (DEAEC). The following are examples of reaction products which can be prepared by variant (b): hydroxyethyl cellulose (HEC) or hydroxypropyl cellulose (HPC). The following are examples of reaction products which can be prepared by variant (c): sulfonoamidoethyl cellulose (SAEC) or cyanoethyl cellulose (CNEC). Mixed ethers of cellulose which can be prepared by the same variant(s) or different variant(s) of those indicated include, for example, methyl hydroxyethyl cellulose (MHEC), ethyl hydroxyethyl cellulose (EHEC), hydroxyethyl hydroxypropyl cellulose (HEHPC), methyl carboxymethyl cellulose (MCMC), hydroxyethyl phosphonomethyl cellulose (HEPMC), or methyl hydroxyethyl hydroxypropyl cellulose (MHEHPC). Within the scope of the statements below, the term "cellulose ethers" is to be understood as meaning both products having a unitary substituent, such as hydroxyethyl cellulose, and products having at least two different substituents, such as methyl carboxymethyl cellulose.
Most of the known processes for the preparation of cellulose ethers are carried out in two main steps:
1. The preparation of the "alkali cellulose".
2. The etherification of the cellulose molecule.
For preparing the "alkali cellulose", cellulose in a finely divided (for example, ground) form is mixed as homogeneously as possible in suitable technical equipment with water and alkali metal hydroxide (in general NaOH, but other bases, such as quaternary ammonium bases, are also possible). The alkali metal hydroxide is used in a solid form or in the form of an aqueous solution. For the etherification reaction itself, and thus for the quality of the end product of the reaction, the uniformity and intensity of the mixing is of decisive importance. Alkalization is generally effected at as low a temperature as possible, for example, room temperature or below, in order to suppress degradation of the polymer (the so-called "ripening"); however, under certain circumstances, for example, the subsequent preparation of low-viscosity cellulose ethers, this degradation may be desirable. An etherifying agent is optionally added as early as the alkalizing step, but in this case the temperature must generally be increased, in order to carry out the actual etherification reaction.
The actual etherifying step is generally run by heating the alkali cellulose produced in the first step, together with the etherifying agent which has previously been added, to temperatures between 30.degree. and 120.degree. C. It is also possible to remove, in advance, part of the water present in the first step. Vigorous mixing in the second step is also very important for the quality of the reaction product and for the cost-effiency of the process, since, for example, it is desirable to have a good yield in the substitution reaction, while employing as small a quantity as possible of the etherifying agent(s).
Both continuous and discontinuous procedures are known for the two reaction steps. In the case of particular reactants, it is also possible to combine the two steps in such a way that pre-alkalization of the cellulose does not take place. Dispersing auxiliaries (suspending agents) are optionally employed in both steps, or at least in one of the two steps, in order to achieve better mixing of the heterogeneous reaction mixture, and for this purpose organic solvents which are either soluble in water or more or less insoluble in water are known from the state of the art; the most frequently used solvents of this kind include isopropanol, tert-butanol, methyl ethyl ketone, or aromatic hydrocarbons, e.g., benzene or toluene.
The state of the art also describes processes for the preparation of water-soluble cellulose ethers, in which an ammonia-activated alkali is employed.
In the paper "The Effect of Activation by Ammonia on the Alkalization and Xanthation of Cellulose" by Schleicher et al., published in "Faserforschung and Textilechnik" ("Fiber Research and Textile Engineering") 24, 1973, number 9, pages 371 to 376, it is stated that after activation of cellulose with liquid ammonia or aqueous solutions of ammonia, conversion to alkali cellulose can be effected at lower NaOH concentrations, as compared with untreated cellulose. In general, activation proceeds at temperatures between 0.degree. and -50.degree. C., for a duration of about 30 minutes; the activating agent is then removed at room temperature and after this, alkalization is initiated. Activation with liquid ammonia, followed by evaporation of the ammonia and drying at room temperature before carrying out the final reaction, is considered to be particularly favorable.
Activation of cellulose with solutions of ammonia in various organic solvents is described by Koura et al. in their paper "Investigations on the Swelling and Dissolution of Cellulose in Mixtures of Liquids Containing Amines", published in "Faserforschung und Textiltechnik" (Fiber Research and Textile Engineering") 24, 1973, number 2, pages 82 to 86. According to this paper, solutions of ammonia in solvents such as water, dimethylsulfoxide, formamide, monoethanolamine, or morpholine can be used to produce effects similar to those achieved with liquid ammonia; this does not apply, however, to solutions of ammonia in alcohols, for example, methanol or ethylene glycol, which do not cause a structural change of the cellulose. Esterification is the only chemical reaction which is carried out with the activated or non-activated celluloses.
The paper "Preparation of Alkali-Soluble Cyanoethyl Cellulose from Preactivated Native Cellulose", by Koura et al., published in "Faserforschung und Textiltechnik" ("Fiber Research and Textile Engineering") 28, 1977, number 2, pages 63 to 65, gives an account of the preparation of cellulose ethers (using cyanoethyl groups as the substituent) from cellulose which has been activated with liquid ammonia. Even at a low degree of substitution of 0.5 or less, the reaction products obtained in the process already have a good solubility in an aqueous NaOH solution. Prior to the etherification reaction, the ammonia is completely removed or washed out, respectively.
As is stated by Schleicher et al. in their paper "Influencing the Reactivity of Cellulose by Activation", published in "Das Papier" ("Paper"), 34th year, number 12, 1980, pages 550 to 555, pretreating with ammonia does not result in any changes or produces only slight changes in the initial velocity of reaction and in the degree of substitution attained, in the case of carboxymethylation and methylation. However, this statement is based on the assumption that there is a change in the distribution of substituents (increased uniformity).
In the paper "The Effect of Preactivation on the Carboxymethylation of Cellulose", by Dautzenberg et al., published in "Acta Polymerica", 31, 1980, number 10, pages 662 to 667, it is pointed out that preactivation of cellulose with NH.sub.3 leads to an improved solubility of the NaCMC produced. Improved solubility is, in particular, observed, if etherification itself proceeds in a cellulose/NaOH/water system, which additionally contains an organic solvent, for example, ethanol. The effect achieved is attributed to a more uniform distribution of the substituent. In carrying out the reaction (a) liquid ammonia is caused to act at -50.degree. C., (b) the ammonia is evaporated, (c) the activated cellulose is alkalized and (d) the alkali cellulose is reacted with the etherifying agent.
According to German Democratic Republic Patent No. 146,462, a process for the preparation of water-soluble NaCMC having a low degree of substitution of 0.5 or less uses preactivated cellulose and includes the steps of (a) activation with liquid ammonia at -50.degree. C., (b) removal of the ammonia by evaporation, (c) alkalization in an NaOH/H.sub.2 O/ethanol system and (d) etherification. In the reaction product, the constituents which are insoluble in water are reduced to less than about 5%, compared with 12 to 18% in reaction products prepared from cellulose which has not been preactivated.
From German Democratic Republic Patent No. 148,342 it is known that preactivation of cellulose with liquid ammonia, in the preparation of NaCMC covering a DS (degree of substitution) range from 0.4 to 0.7, results in a viscosity increase by a factor from 2 to 5. The reaction process corresponds to that described in the above-mentioned GDR Pat. No. 146,462. The aqueous solutions which can be prepared from the NaCMC still contain relatively large amounts of insoluble constituents, ranging from about 4 to 15%.